1. Field of the Invention
Embodiments described herein generally relate to apparatus and method for removing and recovering oil from solids, such as wellbore cuttings.
2. Description of the Related Art
In the drilling of oil and gas wells, drilling fluids or “muds” are used to provide wellbore lubrication, to cool the drill bit, to protect against corrosion, to provide a pressure head to maintain formation integrity, and to remove wellbore cuttings. One type of drilling mud used is oil-based. Oil-based drilling muds are employed in operations where it is desirable to drill at elevated temperatures, improve bore hole stability, control shale sloughing, and control water wetting of the formation such as in clay and some shale formations. Oil-based drilling muds are also desirable in “sour gas” wells and inhibit corrosion and provide superior lubrication of the drill pipe in the wellbore.
A typical oil-based drilling mud includes a diesel, synthetic or mineral oil base, oil soluble emulsifiers, water (often salt water), oil wetting agents such as calcium sulfonates and organic amines to provide for oil wetting of the solids, and additives to control leak-off into the formation.
Drilling muds are typically circulated down the inside of a tubular drill string, outwardly through the drill bit and up the annulus between the drill string and the bore. Drilling muds serve to carry the wellbore cuttings away from the bit and out of the bore hole. These cuttings contain varying amounts of oil retained on the cuttings as a result of the oil used in the manufacture of oil-based drilling muds. Typically, cuttings extracted from the bore hole first go across a shaker, e.g., a shale shaker, to separate the mud from the cuttings. When the oil saturated cuttings are separated from the oil-based drilling muds with the shaker, the cuttings extracted from the fluid phase of the oil-based muds are particulate solids retaining a high percentage of oil (e.g., 18% by weight) on the surface of the solids.
The separated cuttings include all solids that are separated from the oil-based drilling mud circulated through a wellbore during normal drilling operations. Most of the solids comprise the actual wellbore material produced when drilling the formation. Additional solids that may be recovered are materials which are added to the oil-based drilling mud to enhance or maintain the oil-based drilling fluid parameters. Some of these include weighting materials (such as barite, calcium carbonate, etc.), lost circulation materials (such as various fibrous plugging agents), and various chemicals used to maintain the drilling fluid's properties. These additions will result in cutting sizes varying from wellbore cuttings (20 to 100 microns) to weighting materials (e.g., barite) which is finely ground (about <20 microns). Therefore, the wellbore cuttings extracted from the formation will consist of particulate solids having a wide range of sizes which vary from extremely fine reground material to a coarser or larger oil-based cutting.
Wellbore cuttings adhere to the oil, trapping large quantities of oil-based mud in their intergranular spaces and creating environmental concerns regarding disposal of the contaminated wellbore cuttings. For example, uncleaned wellbore cuttings which are dumped can cause substantial pollution as the oil is gradually released from surface adhesion to the wellbore cuttings. Because the industry and the U.S. government want to avoid any such oil pollution, regulations governing the disposal of wellbore cuttings or solids have been promulgated.
In the past, contaminated wellbore cuttings were transported to appropriate disposal sites. Such storage and transportation operations are costly and environmentally undesirable. Typically, oil-contaminated cuttings contain about fifty percent (50%) by volume of oil-based liquid. Because the value of this large volume of entrained oily liquids is considerable, there is a strong economic incentive to recover the entrained oil both for economic as well as environmental reasons. Therefore, there is a need for an apparatus and method of removing and recovering oil from wellbore cuttings which achieves a high oil recovery.